The present invention relates to improvements in rubber compression spring units including spring block elements of the sandwich type comprising layers of rubber bonded to metal end plates and intermediate metal plates, the element forming a resilient block with a laminated structure. The invention relates more particularly to rubber spring units intended for operation chiefly in compression to support a load or a pressure exerted vertically and to work occasionally in shear when, to the main effect of compression, there is added a component directed horizontally and resulting, for example, in a horizontal displacement of the supported load. Such spring units are disclosed, for example, in U.S. Pat. Nos. 3,575,403; 3,471,165; 3,730,509 and 3,504,905.
Spring units of this kind are used, for example, in the suspension of truck bodies wherein the body is supported either directly on the undercarriage chassis (secondary suspension) or on floating bolsters (tertiary suspension). These spring units are placed on each side of the undercarriage chassis wherein each spring unit is between a stationary beam and a pivot arm to form the side bearers for the body on the undercarriage. The pivot arm acts as a reactive force to the load and compresses the spring unit against the stationary beam. In these applications, these rubber spring units have in particular the advantage of preventing the transmission of mechanical and sound vibrations between the undercarriage and the body of the carriage and of reducing, in consequence, the running noises.
When the vertical deflection of the suspension is effected at another part of the suspension, the side bearers are not required to have a large vertical deflection under load but they are required to have a slight stiffness in horizontal shear in the longitudinal direction of the carriage, to allow particularly the rotation of the undercarriage with respect to the body when the truck is moving in a curve, for example. The spring units must also offer a definite resilience in horizontal shear in the transverse direction to allow the displacement and the transverse return of the body with respect to the chassis. This transverse return, in the case of truck bodies, should have a value that is almost constant whatever the load transported by the truck.
It is known that the stiffness in shear of a rubber spring diminishes rather more as the spring is compressed, and that it can even become negative, wherein the spring becomes unstable resulting in buckling. This is a serious drawback to the use of such rubber springs as side bearers of the body when the loading of the body can vary to a high degree between the unladen weight and the full load. In these cases, in fact, it is difficult to produce rubber spring units offering the desired rigidity in shear, not only when the carriage is unladen but also when it is fully loaded or overloaded. In fact, it has been found that in the unladen position of a suspension having a spring unit interposed between a stationary beam and a pivot arm, the inside portion of each spring unit is in compression and the outside portion of each spring unit is in tension.
It has been found that, depending on the length/width ratio and the shear/compression spring rate ratios, buckling of the spring block element will occur at loadings and deflections which typically occurs at about 20% compression deflection. Furthermore, it has been found that in a rubber compression spring unit having two spring block elements, the inboard end of the spring will be compressed to a greater percent of its rubber wall and hence is the point at which buckling occurs. U.S. Pat. No. 3,575,403 discloses a compression spring including spring elements with layers of rubber bonded to intermediate plates characterized in that the compression spring has two spaced spring elements and arranged so that when the spring flexes in compression, the elements tend to approach and rest on each other.